EFFECT OF INCUBATION COUPLED WITH ARTIFICIAL AGING IN T6 HEAT TREATMENT OF A356.2 ALUMINUM CASTING ALLOY
J. Manickaraj, G. Liu and S. Shankar Light Metal Casting Research Centre (LMCRC), McMaster University, Hamilton, ON, Canada Copyright ©2011 American Foundry Society Abstract
A356.2 is one of the most popular Al casting alloys for structural applications in automotive and aerospace sectors. Typically, the cast components from these alloys undergo a T6 heat treatment to significantly improve the mechanical properties. Solution heat treatments, incubation at room temperature, followed by artificial aging at an elevated temperature are the typical steps in a T6 heat treatment process. The incubation process (IP) is typically carried out between 4 to 12 hours on commercial foundry products and typically longer on aerospace castings. There has been a few studies on the mechanisms of precipitation reactions occurring during the incubation and natural aging processes, however all these are for the Al-Si-Mg wrought alloys which are markedly different in composition from Al-Si-Mg casting alloys such as the A356.2. A hypothesis for the sequence of precipitation reaction in the primary Al phase during incubation and artificial aging processes, and the
Introduction
One of the most versatile Al-Si-Mg casting alloys is the A356.2 alloy; widely used for the casting of high strength components in automotive, aerospace and military applica- tions due to its excellent castability, weldability, high strength, pressure tightness and corrosion resistance.1
The alloy is gen-
erally heat treated to provide various combinations of desired mechanical (strength and toughness) and physical properties.
The most common heat treatment for Al-Mg-Si alloys is the T6 treatment, wherein the component is solution heat treated at around 540C (1004F) for about 10 to 12 hours immediate- ly followed by quenching in water maintained at 80C (176F). The component is then left to incubate at room temperature for about 4 to 12 hours followed by artificial aging at around 155C (311F) for about 2 to 12 hours.2,3
The terms natural
aging and incubation are sometimes used interchangeably, but they refer to two different phenomena. Incubation initi- ates the low temperature ordering of Si and Mg atoms from super saturated solid solution in preparation for artificial aging without creating a specific crystal structure. Whereas natural aging is the more complete aging phenomenon creat-
International Journal of Metalcasting/Fall 2011
quantified effect of these aging treatments on the resultant tensile properties of the cast components with A356.2 Al alloy is presented. The results show that depending on incubation time, the formation of individual clusters of Si and Mg, breakdown of the individual-clusters of Mg and formation of the co-clusters of Si and Mg (Mg5
take place. The formation of Mg1.8Si and the final stable Mg2
Si6
of the precipitation reactions were formulated from micro- hardness data on the primary Al phase matrix, tensile properties and information from the prior-art in this field of study. Further, the time of incubation has a significant effect on the resultant mechanical properties because the precipitation reaction during artificial aging is directly affected by the duration of incubation time.
Keywords: aluminum alloy, incubation, natural aging, T6, heat treatment, micro-hardness, tensile properties, A356.2
ing precipitates of Mg and Si atoms having specific crystal structure like those developed during artificial aging but ac- complished over sufficient time at ambient temperature. The rate of quenching after solution heat treatment needs to be optimized to minimize distortion and residual stresses in the component.4
Table 1 presents the standard specification for the T6 heat treatment for components cast in a permanent mold casting process with A356.0.2,3
Specifications do not mention the du-
ration of holding between quenching and loading for the ar- tificial aging treatment at 155C (311F). This holding period subsequent to quenching is termed incubation and is gener- ally carried out at room temperature. Typically, permanent mold castings of A356.2 experience an incubation time of 4 to 10 hours, at room temperature.5
The incubation process is widely used in Al wrought alloys. In the Al 6xxx series alloy, Mg and Si atoms are found to organize into specific individual-clusters and co-clusters and affect the precipitation mechanism during the artificial ag- ing treatment.6-8
The artificial aging treatment is carried out 17 ) would Si phases occurs during artificial aging. The sequence
Typically, water at about 80C (176F) is used for quenching components after solution treatment.
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